1. Field of the Invention
This invention relates to an image processing apparatus and method. More particularly, the invention relates to an image processing apparatus and method for processing input image data in such a manner that a predetermined output characteristic is obtained.
2. Description of the Related Art
The gamma table in an image forming apparatus is engineered in such a manner that the design center value of an output density characteristic under conditions in which a conversion is not being applied will be converted to a desired output density characteristic. However, owing to error attributable to the variety of component parts that constitute an image forming apparatus, the finished products exhibit output density characteristics that differ from one apparatus to the another.
FIG. 1 is a diagram showing an example of the output density characteristic of such an apparatus. Here an image signal value is plotted along the horizontal axis and output density along the vertical axis. The curve indicated at 2-a represents the design center value of the output density characteristic, and the curve indicated at 2-b represents an output density characteristic that has deviated from the design central value.
Recent technological advances have led to the appearance of many image forming apparatus equipped with photographic and other modes for reproducing high-definition images exhibiting a high degree of tonality. Individual differences in output density characteristics in such apparatus greatly detract from image quality. Accordingly, methods of printing an image based upon a prescribed image signal, measuring the output density characteristic of the apparatus by reading the image and then correcting the output density that has been measured have been disclosed in U.S. Pat. No. 5,258,783 and U.S. Pat. No. 4,888,636.
Though a variety of these correction methods have been developed, it is generally necessary to implement them upon balancing cost against required image quality in the case of a monochromatic image forming apparatus. More specifically, since the output density characteristic in the vicinity of intermediate density is linear, as shown in FIG. 1, a primary conversion which makes this linear portion conform to the design center value is applied to the image signal of an apparatus whose output density characteristic has deviated. This shall be referred to as a "linear image adjustment" below. This adjustment makes it possible to obtain the desired output density characteristic irrespective of individual differences in the image forming apparatus.
However, a problem which arises is that when a linear image adjustment is carried out, there are instances where the output density characteristic exhibits a characteristic different from the design center value in a density area other than that where the output density characteristic is linear. In such instances there is no problem in terms of density reproducibility of image halftones but there are cases where a phenomenon occurs in which a contour that is originally non-existent appears in portions where density is low. This shall be referred to as a "false contour phenomenon" below. In particular, when importance is placed upon gradation, as in the case of the photograph mode, image quality deteriorates markedly if the false contour phenomenon occurs.